Hello Uwe,
Although you can set navigation to sequential when all questions are displayed on the same page, sequential navigation only makes sense if the questions are displayed on different pages, usually one question per page.
When the navigation method is set to "Sequential" and students submit their attempt, the "Return to attempt" button, on the "Summary of attempt" page, returns students to the last page they viewed. If the last page they viewed is the last question in the quiz, students will not be able to see any of the previous questions since navigation is sequential.
To prevent students from viewing again the last question of the quiz, you can try to trick them by putting a description question at the very end of the quiz. If students view this last description question, when they submit their attempt and return to it, they will only be able to see the last description question and no other questions. This last description question could contain a message like "Important - You have tried all questions. You must now click on the "Finish attempt..." button.
If this works for you, you don't need a script.